Virtual Plasmonic Dimers for Ultrasensitive Inspection of Cluster–Surface Coupling

Plasmonic sensors owe their sensitivity to the susceptibility of collective electron modes to tiny changes of the local environment. Simple sensors are based on metal nanoparticles where spectral shifts of the resonance are frequently treated in view of an “effective” dielectric medium surrounding each particle. Using single-object spectroscopy probing a large number of silver particles on Si(111)-(7 × 7) in a photoemission electron microscope (PEEM), we show that this picture breaks down because of the formation of image dipoles in the substrate. Even at particle sizes as small as 10 nm, that is, small compared to the excitation wavelength, the dipole approximation is no longer valid because the system inherently creates higher order multipole modes. To validate this scenario, we compare plasmon energies, lifetimes, and their variability to the case of a surface with lower permittivity. Model calculations based on generalized Mie theory reveal that the formation of multipole modes is extremely sensitive to the local geometry on the scale of one angstrom, in particular to the intradimer separation. Hence, such systems may enable novel ultrasensitive plasmonic detection schemes such as high-precision plasmonic rulers and probes for molecular interfaces or spacer layers.